This invention relates to an electronic device having a protective circuit for protecting its circuit elements from breaking down due to abnormally high power voltage.
Such a protection function is especially necessary for the electronic devices in an automobile. Those electronic devices are usually energized by a storage battery which is normally charged by a dynamo rotating in synchronism with the engine. When the engine is running, the output power voltage of the dynamo is applied to both the electronic device and the storage battery. Accordingly, when the connection wire to the storage battery is accidentally disconnected from the battery, an abnormally high surge voltage appears at the output of the dynamo for a short time immediately thereafter. This causes a breakdown of electrical elements in the electronic devices.